What entropy means for a password
Entropy, measured in bits, is how unpredictable a password is. If each of L characters is chosen uniformly at random from N possibilities, entropy = L x log2(N). A 16-character password drawn from the 86 characters this tool offers (26 + 26 + 10 + 24 symbols) has 16 x 6.43 = about 103 bits. Twelve characters give 77 bits; eight give 51. Every extra bit doubles the number of guesses an attacker needs on average.
The crack-time estimate assumes an offline attack at 10 billion guesses per second - what a mid-size GPU rig manages against fast hashes like MD5 or NTLM - and reports the average time, which is half the keyspace. At that rate 40 bits fall in about a minute, 60 bits take about 2 years, and 80 bits take about 2 million years. Sites that hash properly with bcrypt or Argon2 slow attackers by a factor of 10,000 or more; sites that store plaintext make entropy irrelevant. Since you rarely know which you are dealing with, plan for the worst.
The formula only holds for truly random choices. P@ssw0rd2024! uses every character class, but it is a dictionary word with predictable substitutions and a year, and cracking tools try exactly those patterns first. That is why a generator beats anything you make up.
Character passwords vs passphrases
Character mode uses crypto.getRandomValues with rejection sampling so every character is equally likely (a plain modulo would slightly favor some characters). It guarantees at least one character from each set you check, which satisfies the "must contain uppercase, a digit and a symbol" rules most sites enforce, then shuffles so the guaranteed characters are not always first. "Exclude ambiguous" removes 0, O, 1, l and I for passwords you may read over the phone or type from paper.
Passphrase mode follows the diceware idea: pick words at random from a fixed list and join them. The built-in list has 256 common, easy-to-spell nouns, so each word contributes exactly 8 bits; five words give 40 bits, six give 48, and the optional two-digit number adds another 6.6. That is plenty for accounts protected by rate limiting and two-factor authentication, and far easier to remember than x7#Kq2!v. For a password-manager master password or full-disk encryption, where offline attacks are realistic, use a longer list such as the EFF's 7,776-word diceware list (12.9 bits per word) and six or more words - or simply use a long character password and let the manager remember it.
Using the results
Use a different password for every site so one breach does not open the rest, and store them in a password manager (Bitwarden, 1Password, KeePass or the one built into your browser and phone). Turn on two-factor authentication for email and financial accounts. Passwords are generated in your browser and never transmitted; reload the page and they are gone.